SK37 and SK39 primers in polymerase chain reactions (PCR), we amplified and cloned, from an HIV-I seropositive mother and her child, a portion of HIV-related gag sequence (500 bp).The sequences cloned were very similar to each other, suggesting the possibility of vertical transmission. The few subtle differences between the two sequences suggests they may be considered a quasispecies. These clones showed a homology score of 82.7-87.2% with gag sequence of prototype HIV-1. The divergence was particularly prominent in the first 5'proximal 200 bp which gave only 79-80% homology with the corresponding domain of prototype virus sequences. Thus the clones represent variants that are markedly divergent from the prototype HIV-I stains. There was an early appearance of stop codons in the cloned sequences, suggesting the presence of defective viral genomes. The ability of cellular factors to transactivate the HIV-1 LTR was investigated in two promonocytic cell lines, U937 and HL-CZ, which represent cells whose differentiation has been arrested at different stages. The two lines differed in their constitutive transactivating abilities and the extent with which they responded to enhancement by TPA and TNF. They also showed differences in their response to deletion of a negative regulatory element from the LTR. These differences may be related to variations in the efficiency with which the latent HIV genome is activated.